Ask HN: Where is the best place to find HS/College Brand Ambassadors?

I've posted ads on Craigslist and at schools but am having a difficult time finding brand ambassadors (similar to bzzagent.com) - either paying directly or giving swag. My startup is directed at students so that is not the problem. The students I've encountered so far have been very motivated and then did abut ultimately did nothing. I'm in the US. Any ideas would be appreciated.